In the 1890s, the story goes, a man by the name of William Radam became a millionaire by selling bottles labeled "Microbe Killer Number One." Unbeknownst to his customers, his concoction contained only water and a drop of wine, a potent recipe for snake oil, perhaps, but not an antibiotic. More than a century later, science and the marketplace have conspired to give us countless (albeit generally more effective) products for killing off germs. From antibacterial soaps and cleaners to antibacterial cutlery and even children's toys, the number of these products on the market has soared in recent years.

The urge for sterilized purity is understandable enough: Deaths caused by infections in the United States rose 58 percent between 1980 and 1992, microbes are developing antibiotic-resistant strains at a frightening rate, and news of outbreaks of Ebola, hanta virus, and flesh-eating strep infections not to mention the more common E. Coli and salmonella has sent us on a fearful cleaning spree, as we try to keep our lives as germ-free as possible. But, as Garry Hamilton explains in the British journal New Scientist (July 18, 1998), "today's squeaky clean world" could be making us ill.

According to an increasing number of scientists, our "growing separation from dirt and germs" may be behind the rapid rise in allergic diseases, such as asthma, and perhaps even autoimmune diseases, such as diabetes and rheumatoid arthritis. "Wherever you look," writes Hamilton, "allergic diseases seem to follow hard on the heels of wealth and modernization"; asthma, hay fever, and eczema are all on the rise, and doctors report seeing more children at a younger age developing insulin-dependent diabetes.

According to a new theory, dubbed the hygiene hypothesis, these trends are the result of our society's aversion to dirt and muck. "During most of evolution the immune system was bombarded with dirt and germs from the moment each newborn infant hit the cold light of day," Hamilton explains. Just as the brain cells of a baby need stimulation to make the right connections, the argument goes, the immune system needs proper stimulation to develop properly. According to the hygiene hypothesis, exposure to the microbes that live in dirt and grime helps the immune system defend against certain diseases, particularly asthmaówhich afflicts 14 million people in the United States, killing 5,000 every year.